The reason your sidebar has fallen and is displaying below your posts is either due to copying and pasting or it can also be a result of importing text containing invalid HTML. Typically this is caused by an open HTML tag in one of your posts.
See: Themes » Layout Issues > Sidebar

Posts > All Posts, change all your latest posts to Draft, click Update, change them back to Published, click Update again. (You can edit all of them at once: tick the square in front of their titles then select Edit from the Bulk Actions dropdown and click Apply.)